In mobile telephony systems for instance, information is transferred from a base station to a plurality of mobile stations. The information is transmitted on different carrier waves and it is desirable, for cost reasons, that several carrier waves can be power amplified simultaneously in one and the same power amplifier.
In a power amplifier that is based on a traditional feedforward technique, a radio signal that includes at least two modulated carrier waves is amplified in a first non-linear amplifier, wherewith intermodulation products arise in the output signal of the first amplifier. A small part of the output signal from the first amplifier is added in reverse or opposite phase to a forwardly fed part of the input signal of the first amplifier, wherein the reverse phase addition results in a signal that contains only the earlier mentioned intermodulation products. A compensation signal is generated by amplifying this last-mentioned signal to a suitable power level, in a linear amplifier. A power amplifier output signal is generated by reverse phase addition of the compensation signal to the output signal from the first amplifier, thereby suppressing the intermodulation products in the output signal from the power amplifier.
One problem encountered with power amplification that is based on a traditional feedforward technique arises as a result of the connection to the output of the first amplifier of a number of components that cause power losses and therewith impair the efficiency of the power amplifier.
German Patent Specification DE 27 18 172 teaches an amplifier which when amplifying an input signal that contains several frequencies suppresses intermodulation products in the amplifier output signal. This amplifier is divided internally into a first and a second signal path. The first signal path includes a first non-linear amplifier and the second signal path includes a second non-linear amplifier that is connected in series with an attenuator. The amplifier divides the input signal on the two signal paths, wherewith the first and the second amplifier both generate a respective output signal that includes both a desired signal component corresponding to the input signal and intermodulation products that arise in the amplification process. Due to higher amplification of the input signal in the second amplifier, the output signal from the second amplifier contains much stronger intermodulation products than the output signal from the first amplifier. The output signal from the second amplifier is attenuated in the attenuator, so that the intermodulation products in the attenuated signal are equally as large as the intermodulation products in the output signal from the first amplifier. The amplifier output signal is generated by reverse phase addition of the output signal from the first amplifier to the attenuated output signal from the second amplifier.
One problem encountered with the amplifier taught by DE 27 18 172 resides in the significant losses in power that are experienced. One reason why these power losses occur is because the input signal is amplified much more strongly in the second amplifier than in the first amplifier so that the output signal from the second amplifier can then be attenuated, such that the intermodulation products that are generated in the first and the second signal paths in the subsequent reverse phase addition process will neutralise each other.
Power losses are also caused because the signal obtained subsequent to attenuation in the second signal path still contains a relatively large part of the desired signal component, and hence this part of said signal will cause a power reduction of the desired output signal from the amplifier in said reverse phase addition process.